1. Field of the Invention
The present invention relates to a semiconductor photosensor device and a portable terminal unit, and particularly relates to a semiconductor photosensor device which realizes an improvement in responsibility in a shift from a standby state to an operating state as well as a reduction in power consumption, and a portable terminal unit using the semiconductor photosensor device.
2. Description of the Related Art
A semiconductor photosensor device is a photosensor which provides a linear output according to its ambient illumination (lightness), and mainly in a portable terminal unit, it is used for the ON/OFF control of a liquid crystal backlight and an LED provided in a key operation section according to the ambient illumination (lightness). It is used as a sensor which reduces unnecessary power consumption, for example, by turning off the backlight and the LED of the key operation section when its surroundings are light, and when its surroundings are dark, turning them on, or turning them on after brightness is adjusted.
In the portable terminal unit, for example, as shown in FIG. 1, an output of the semiconductor photosensor device is read in predetermined timing, for example, when a key operation is performed, and the brightness of the backlight and the LED is adjusted according to the output of the semiconductor photosensor device.
Specifically, in the example in FIG. 1, when no key operation is performed (step S10: No), the current states of the backlight and the LED of the key operation section are maintained (step S12). On the other hand, when a key operation is performed (step S10: Yes), the output of the semiconductor photosensor device is read (step S14). When the output of the semiconductor photosensor device is low (step S16), the LED of the key operation section is turned on, and the backlight is also turned on (step S18). When the output of the semiconductor photosensor device is medium (step S20), the LED of the key operation section is turned off, but the backlight is turned on (step S22). When the output of the semiconductor photosensor device is high (step S24), the LED of the key operation section is turned off, and the backlight is also turned off (step S26).
FIG. 2 is a block diagram showing the circuit configuration of a related semiconductor photosensor device. As shown in FIG. 2, the related semiconductor photosensor device includes a photodiode current arithmetic circuit 10 connected between a power supply terminal VCC and a ground terminal GND and plural current amplifiers 12, 14, and 16 connected in series. An output of the photodiode current arithmetic circuit 10 is amplified by the series-connected current amplifiers 12, 14, and 16, and outputted as an output current from an output terminal OUT.
FIG. 3 is a diagram showing an example of the concrete circuit configuration of the photodiode current arithmetic circuit 10, and FIG. 4 is a diagram showing the cross-sectional structure of photodiodes PD1 and PD2 in FIG. 3. The photodiode current arithmetic circuit 10 such as shown in FIG. 3 and FIG. 4 is disclosed, for example, in Japanese Patent Laid-open No. 2002-217448. As shown in FIG. 3, the photodiode current arithmetic circuit 10 includes transistors Q1 to Q4 in addition to the photodiodes PD1 and PD2. An n-times current mirror circuit includes the transistors Q1 and Q2, and an m-times current mirror circuit includes the transistors Q3 and Q4.
As shown in FIG. 4, cathode electrodes of the photodiodes PD1 and PD2 are formed by a common N-type semiconductor region. The anode side of the photodiode PD2 is connected to the ground GND via a P-type semiconductor region, and the anode side of the photodiode PD1 is connected to an anode electrode via a P-type semiconductor region.
FIG. 5A to FIG. 5E are diagrams showing operation waveforms of the semiconductor photosensor device shown in FIG. 2. As shown in FIG. 5B, in the related art, the voltage is constantly supplied to the power supply terminal VCC. Therefore, as shown in FIG. 5C, the output current from the output terminal OUT also flows constantly, and hence, as shown in FIG. 5D and FIG. 5E, both current consumption and power consumption are constant.
If the key operation of the portable terminal unit is performed in predetermined timing as shown in FIG. 5A, the output current of the semiconductor photosensor device is read after a predetermined period of time from when the key operation is operated, and based on the result of this reading, the LED of the key operation section is turned on and/or the backlight is turned on.
However, in such a semiconductor photosensor device, a power supply voltage is constantly supplied to the power supply terminal VCC, whereby if the photodiode current arithmetic circuit 10 is exposed to light, a current corresponding to the light is always outputted. Therefore, electric power is consumed even during a period when light illumination need not be detected by the semiconductor photosensor device.
To avoid the above situation, in the related art, when the semiconductor photosensor device is used, as shown in FIG. 6A to FIG. 6E, the supply of the power supply voltage to the power supply terminal VCC is constantly stopped, and at a point in time when the key operation of the portable terminal unit is performed, the supply of the power supply voltage to the power supply terminal VCC is started. Then, after the output current is read after a predetermined period of time, the supply of the power supply voltage to the power supply terminal VCC is stopped. In so doing, when the key operation is performed, the illumination of the portable terminal unit can be detected, and in addition, a reduction in power consumption can be realized.
However, if the power supply voltage is supplied to the power supply terminal VCC in such a manner as shown in FIG. 6A to FIG. 6E, a current flows to the photodiode current arithmetic circuit 10 and the first stage current amplifier 12 after the supply of the power supply voltage to the power supply terminal VCC is started, and hence charge and discharge of parasitic capacitances of respective elements of the photodiode current arithmetic circuit 10 and the current amplifier 12 are performed by this current. In this semiconductor photosensor device, charging/discharging currents of these parasitic capacitances are supplied by a photocurrent flowing to the photodiodes PD1 and PD2 in FIG. 3.
This photocurrent is a minute current of about several nanoamperes (nA) at an illumination of about 100 lux, whereby the charge and discharge of the respective parasitic capacitances require a lot of time. Moreover, in this semiconductor photosensor device, the photocurrent changes according to illumination, whereby the lower the illumination, the smaller the photocurrent becomes, and consequently, the charge and discharge of the respective parasitic capacitances require more time.
Hence, there is a problem that a time Twait from when the key operation is performed until the output current of the semiconductor photosensor device is read is as long as several tens of milliseconds to 100 milliseconds. Moreover, there is a problem that a user has to use the portable terminal unit during this time without the brightness adjustment of the LED of the key operation section and the backlight of the portable terminal unit, for example, in a state in which the picture is hard to see. Therefore, there is a strong demand for a reduction in the time from the instant of the key operation or the like until the output current of the semiconductor photosensor device is read as well as a reduction in power consumption.